[FIX] web_editor: unexpected copy paste behaviour of link

Current behaviour before commit:

-When pasting copied content from editor inside
link inserts text with HTML content, in result
the pasted content seems isolated from the link.

e.g. <a href="#">te[]st</a>
	+ pasting <h1>123</h1>  <=>
	<a href="#">te<h1>123</h1>st</a>

Desired behaviour after commit:

-Now only text content is pasted which makes
pasted content as a part of the link.

e.g. <a href="#">te[]st</a>
	+ pasting <h1>123</h1>  <=>
	<a href="#">te123st</a>

closes odoo/odoo#125776

Signed-off-by: David Monjoie (dmo) <dmo@odoo.com>